1. Field of the Invention
The present invention relates to an inverter which supplies driving voltage to a fluorescent lamp or the like, and particularly to a dimming control technique for adjusting the luminance of a fluorescent lamp.
2. Description of the Related Art
In recent years, liquid crystal display TVs, which provide a TV having a thin shape and a large size, are becoming popular as replacements for CRT-based TVs. Liquid crystal display TVs include multiple cold cathode fluorescent lamps (which will be referred to as “CCFLs” hereafter) or external electrode fluorescent lamps (which will be referred to as “EEFLs” hereafter) arranged on the back face of a liquid crystal panel on which video images are to be displayed, which are used as light-emitting backlights.
The CCFL or EEFL is driven using an inverter (DC/AC converter) which boosts DC voltage of around 12 V, and which outputs the voltage thus boosted in the form of AC voltage, for example. The inverter converts the current flowing through the CCFL into voltage, and returns the voltage thus converted to a control circuit as a feedback voltage, thereby controlling the ON/OFF operation of a switching element based upon this feedback voltage. For example, a CCFL driving technique using an inverter is disclosed in Patent document 1.
With such an arrangement, in some cases, in order to provide a dimming function, i.e., in order to adjust the luminance of a fluorescent lamp, the control circuit for the inverter has a dimming function. There are two types of dimming operations. One is a dimming operation set by a manufacturer that has designed an apparatus mounting a fluorescent lamp and an inverter, and the other is that set by the user when the user uses this apparatus. Examples of such dimming methods include an analog dimming control method (current dimming control method) in which the current flowing through the fluorescent lamp (which will be referred to as “lamp current” hereafter) is controlled, and a burst dimming control method in which the fluorescent lamp is controlled so as to provide intermittent light emission.
[Patent Document 1]
    Japanese Patent Application Laid Open No. 2003-323994
The control circuit generates a pulse modulation signal with a duty ratio that changes according to an error voltage which is the difference between a feedback voltage that corresponds to the lamp current and a predetermined dimming reference signal so as to control the ON/OFF period of the switching voltage to be supplied to a transformer. In the analog dimming method, the lamp current is adjusted by changing the dimming reference voltage. In general, with conventional arrangements, a component external to the control circuit generates the dimming reference voltage in the form of an analog signal, and supplies the dimming reference voltage thus generated to the control circuit.
In some cases, in the analog dimming method, the luminance is switched in a discrete manner. In this case, there is a greater demand for relative precision of the luminance, rather than for absolute precision of the luminance. That is to say, such an arrangement requires relative precision among multiple dimming reference voltages on a voltage-level basis. In a case in which such multiple dimming reference voltages are generated at a circuit external to a control circuit as in conventional arrangements, it is difficult to generate the dimming reference voltages with high relative precision.